Most ceramic filters, hybrid ICs and ceramic capacitors (as well as many other electronic devices) include a sheath covering the internal electronic element to protect the device from external shock. The sheath is typically made from a thermosetting resin which is resistant to both heat and physical shock. An exemplary structure is shown in FIG. 11. The device 20 includes an electronic element body 21, lead terminals 25 which are electrically connected to the body 21, and an outer sheath 22 covering the body 21. The outer sheath 22 is formed of a thermosetting resin.
The outer sheath 22 is typically formed by dipping the body 21 into a resin paste (prepared by dispersing a coating material of a thermosetting resin in an organic solvent) to adhere the resin paste to the body 21 and then drying the paste and removing the organic solvent from the resin paste through evaporation in air at room temperature. These steps are repeated until a desired thickness of the thermosetting resin has been applied to the electronic element body 21. Next, the thus-coated electronic device 20 is heated at a predetermined temperature to cure the thermosetting resin with the result that the electronic element body 21 is protected with the cured resin.
When the electronic device 20 has vibrating components such as electrodes 24, an empty space 23 must be formed around each such vibrating element to ensure that it is free to vibrate. To this end, a thermo-subliming material (such as wax) is applied to the region corresponding to the empty spaces 23 prior to applying a thermosetting resin onto the body 21. Then, the body 21 is processed as described above. The thermo-subliming material has a sufficiently low melting point that it will melt during the heating step used to cure the thermosetting resin, will evacuate the area corresponding to the empty spaces 23 and will be absorbed in the pores of the outer sheath 22.
The conventional electronic device 20 and the method for producing it have the following drawbacks.
The porosity of the thermosetting resin used for forming the outer sheath provides a good impact resistance, makes it easy to mark the surface of the outer sheath and absorbs the wax or other thermo-subliming material which has been evaporated from the empty spaces 23. However, it exhibits a poor moisture resistance, with the result that the characteristics of the electronic devices are often degraded.